PEX-A (PE-Xa, PEXa)
PEX-A is produced by the peroxide (Engel) method. This method
performs "hot" cross-linking, above the crystal melting point. However,
the process takes slightly longer than the other two methods as the
polymer has to be kept at high temperature and pressure for long periods
during the extrusion process. The cross-linked bonds are between
carbon atoms.
PEX-B (PE-Xb, PEXb)
The
silane
method, also called the "moisture cure" method, results in PEX-B. In
this method, cross-linking is performed in a secondary post-extrusion
process, producing cross-links between a cross-linking agent. The
process is accelerated with heat and moisture. The cross-linked bonds
are formed through silanol condensation between two grafted
vinyltrimethoxysilane (VTMS) units, connecting the polyethylene chains
with C-C-Si-O-Si-C-C bridges. After installation, PEX-B have the same
properties as PEX-A.
PEX-C (PE-Xc, PEXc)
PEX-C is produced through
electron beam processing,
in a "cold" cross-linking process (below the crystal melting point). It
provides less uniform, lower-degree cross-linking than the Engel
method, especially at tube diameters over one inch (2.5 cm). When the
process is not controlled properly, the outer layer of the tube may
become brittle. However, it is the cleanest, most environmentally
friendly method of the three, since it does not involve other chemicals
and uses only high-energy electrons to split the carbon-
hydrogen bonds and facilitate cross-linking.
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